1. Field of the Invention
The present invention pertains to pyrotechnic bodies, preferably circular wafers and most preferably washer-shaped discs, made of conventional propellant or gas generant compositions which are fabricated to have a special or unique geometry. The inflator produces a gas which, after filtering out condensed phase products, is preferably used to inflate a gas bag which serves as a vehicle occupant cushion during a collision. More particularly this invention relates to a novel geometry for the generant body wherein a channel region is integrally formed (molded) into one or both side or faces of the body, preferably both.
Even though the generant bodies of this invention are especially designed and suited for creating gas for inflating passive restraint vehicle crash bags, it is to be understood that they would function well in other less severe inflation applications, such as aircraft slides, inflatable boats and inflatable lifesaving buoy devices; would more generally find utility any place a low temperature, non-toxic gas is needed, such as for a variety of pressurization and purging applications as, for example, in fuel and oxidizer tanks of rocket motors; and would be useful as a propellant for recoilless rifles and missile systems, particularly in in-tube rocket motors of the short-burning, high-mass type as disclosed in U.S. Pat. No. 3,886,006.
2. Description of the Prior Art
Automobile gas bag systems have been developed to protect the occupant of a vehicle, in the event of a collision, by rapidly inflating a cushion or bag between the vehicle occupant and the interior of the vehicle. The inflated gas bag absorbs the occupant's energy to provide a gradual, controlled deceleration, and provides a cushion to distribute body loads and keep the occupant from impacting the hard surfaces of the vehicle interior.
The use of such protective gas-inflated bags to cushion vehicle occupants in crash situations is now widely known and well documented.
The requirements of a gas generant suitable for use in an automobile gas bag device are very demanding. The gas generant must have a burning rate such that the gas bag is inflated rapidly (within approximately 150 milliseconds). The burning rate must not vary with long term storage (aging) or as a result of shock and vibration during normal deployment. The burning rate must also be relatively insensitive to changes in humidity and temperature. When pressed into wafers, cylinders, discs or whatever shape, the hardness and mechanical strength of the bodies must be adequate to withstand the mechanical environment to which they may be exposed over the expected inflator system lifetime of at least ten years without any fragmentation or change of surface area. Excessive breakage of the bodies could potentially lead to system failure where, for example, an undesirable high pressure condition might be created within the gas generator device, possible resulting in rupture of the pressure housing.
The gas generant must efficiently produce cool, non-toxic, non-corrosive gas which is easily filtered to remove solid and liquid combustion by-products, and thus preclude damage to the inflatable bag or to the occupant of the automobile.
The requirements as discussed in the preceding paragraphs limit the applicability of many otherwise suitable compositions and shapes thereof from being used in automotive air bag gas generators.
Inflators such as shown in commonly assigned U.S. Pat. Nos. 4,005,876, 4,296,084 and 4,547,342 contain gas generant in the form of pressed pellets or tablets (similar in shape to ALKA-SELTZER.RTM. buffered aspirin) which are randomly packed into the inflator combustion chamber. While such an arrangement may be satisfactory for some purposes, the tablets present a high initial surface area for burning and thus do not provide as soft of an inflation onset as may be desired and achieve low volumetric loading fraction due to the volume lost in the interstitial spaces between tablets.
To achieve slower inflation onset and increase volumetric loading fraction, it has been proposed in U.S. Pat. Nos. 3,901,530 and 4,131,300 to form the pyrotechnic combustible material for inflators in the form of separate discs arranged side by side with separator means disposed between adjacent discs to facilitate quick and uniform combustion of the material.
It has been proposed in U.S. Pat. No. 4,200,615 to fabricate a propellant grain from a plurality of washer-shaped discs 26 arrayed side by side on a linear igniter 8 in an inflator combustion chamber.
It has been proposed in U.S. Pat. No. 4,158,696 to press the propellant into wafers and to configure each wafer in a manner to allow space for a propellant powder to be uniformly placed between the wafers to cushion each wafer from shock and vibration and to insure near instantaneous ignition of all wafers. The disposition of propellant powder between the wafers, however, may undesirable result in a fast onset of burning thus producing too high of an initial pressure peak as well as a decreased burn surface neutrality.
More recently it has been proposed in commonly assigned U.S. Pat. Nos. 4,890,860 and 4,998,751 to fabricate an inflator grain by assembling a plurality of washer-shaped propellant wafers or discs in alternating relationship with a plurality of similarly shaped meshed cushion members which are held in compression in the inflator combustion chamber to achieve improved performance.
It is also known from U.S. Pat. No. 4,817,828 that an inflator grain or material may be formed by assembling a plurality of cylindrically shaped grains (FIG. 5), each having an array of axially alignable central and outer peripheral gas passages. The end surfaces of the grains are provided with peripherally located projections (FIGS. 5 and 8) which abut each other thereby acting as standoffs or spacers thus allowing radial gas flow between grains. The plurality of grains require a rather precise and complex assembling operation involving additional features and components to insure proper alignment of the standoff pads and gas passages. This alignment is achieved by placement of a multi-arm retainer device (FIG. 9) about the overall grain which engages V-shaped notches (FIG. 6 and FIG. 7) in the outer sides of each grain. The retainer becomes a permanent part of the inflator.
In FIGS. 5 and 6 of U.S. Pat. No. 4,919,897 it is also known to provide washer-shaped gas generant discs or moldings with projecting knubs or islands on each side whereby a spacer zone (see FIG. 1) is created between stacked disc-shaped moldings thereby attaining improved combustion of the discs. U.S. Pat. No. 5,551,343 discloses a gas generant wafer having a plurality of alike, flat-surfaced projections equally spaced and radially equidistant about the periphery of one or both faces of the wafer.
Current generation flat geometry washers sometimes fracture during handling and processing. This fracturing causes increased scrap and waste. Additionally, the performance of the flat generant wafers is regressive, i.e., the combustion pressure of the inflators using these wafers decreased with burn time. Regressive burning wafers are less desirable because this lowers the average combustion pressure of the inflators, slowing the gas bag inflation down. By utilizing a more neutral burning wafer, the average combustion pressure of the device can be increased, and faster deployment results.